The present invention relates generally to color imaging assemblies which employ multiple, spectrally selective, reflective layers for generating spacially separated, color component images of an object on an image plane, and more particularly to arrangements of the spaced-apart reflective layers which optimizes component beam imaging and registration with sensor arrays positioned on the image plane.
The phrase "beam of light" is sometimes narrowly defined to mean a bundle of parallel light rays such as those generated by a collimated light source. The phrase "beam of light" may also be more broadly defined to mean any narrow shaft of light having light rays traveling in the same general direction. Used in this broader sense, the light which emanates from an object and passes through the aperture of an imaging lens as well as the converging cone of light which emerges from the lens and which is focused on an image plane may be collectively referred to as a "beam of light." When the phrase "beam of light" is used herein, it is to be understood that this broader meaning is intended.
Vincent al., U.S. Pat. No. 4,709,144 Vincent et at., U.S. Pat. No. 4,870,268, which are hereby specifically incorporated by reference for all that is disclosed therein, describe a number of different dichroic composites which are used in beam splitter assemblies and beam combiner assemblies. An optical scanner which employs a beam splitter is described in commonly assigned U.S. patent application Ser. No. 383,463 filed July 20, 1989, for OPTICAL SCANNER of David Wayne Boyd which is hereby specifically incorporated by reference for all that it discloses. A component beam path length compensator is described in commonly assigned U.S. patent application Ser. No. 498,865 filed Mar. 23, 1990, for BEAM SPLITTER/COMBINER WITH PATH LENGTH COMPENSATOR of Michael John Steinle, which is hereby specifically incorporated by reference for all that it discloses.
Certain prior art beam splitter assemblies which are disclosed in U.S. Pat. Nos. 4,709,144 and 4,870,268 will now be briefly described with reference to FIGS. 1-4.
FIG. 1 is a schematic side elevation view of a line-focus-type color imaging assembly comprising a line object 1 which originates a polychromatic light beam 4 which passes through an imaging lens 6 which is adapted to focus a line image of the line object on an image plane II located at a fixed optical path length distance from the imaging lens 6. The light beam 4 impinges upon a dichroic beam splitter 56 which splits the polychromatic light beam 4 into spectrally and spacially separated color component beams 8, 9, 10 which provide focused color component images of the line object on a monolithic photosensor unit 11, FIGS. 1 and 2, positioned at the image plane II.
FIG. 1 illustrates the manner in which two optically flat transparent optical support media 60 and 62 can be attached to provide three substantially equally spaced dichroic coatings to produce three substantilly parallel optical component beams 8, 9, 10 that are both spacially and spectrally separated. The optical separator 56 consists of precisely ground and polished glass plates 60 and 62 coated on one or both faces with dichroic coatings 50, 52 and 54. At each dichroic coating 50, 52 and 54, incident light is either reflected or transmitted according to wavelength with negligible absorption loss. The composition of the dichroic coatings 50, 52 and 54 can be designed for accurate bandpass filtration.
The plate 60, shown in FIG. 1, is designed such that incident light striking dichroic coating 50 at 45.degree. reflects blue light (approximately 400-500 nm) while transmitting red light and green light.
Plate 62, shown in FIG. 1, is coated on both faces with dichroic coatings 52 and 54 such that an incident polychromatic light beam 4 striking a first dichroic coating 52 at nominally 45.degree. reflects the red spectral band (e.g., 600-700 nm) while transmitting the green band. The green light striking a second dichroic coating 54 and having an optical axis oriented nominally 45.degree. from the dichroic coating is reflected. The reflected green light is caused to pass back through the glass plate 62 and through the other dichroic coatings 52 and 50 at a 45.degree. angle. As shown in FIG. 1, each of the color components 8, 9 and 10 of the incident light are reflected at 90.degree. to incoming beam 4. The reflected red and green components 9 and 8 are parallel and separated from each other by a distance determined by the glass plate 62 and dichroic coating thickness 52, the index of refraction of plate 62, and the angle of incidence. Similarly, the blue and red components 10 and 9 are separated by a distance determined by the thickness of the glass plate 60, dichroic coating 50, the index of refraction of the plate 60 and the angle of incidence.
A mirror coating could be substituted for the third dichroic coating 54, since only the third remaining color component reaches that coating interface.
A suitable photosensor unit 11 for use with optical separator 56 is shown in FIG. 2. Photosensor 11 may be a single chip, single package solid state device having three linear photosensor arrays, 12, 13 and 14, precisely aligned and spaced to coincide with the focused line images formed by beams 8, 9 and 10, respectively, shown in FIG. 1.
As illustrated in FIG. 1, light in each of the color component beams 8, 9, 10 travels a different optical path length through the beam splitter 56. As a result in the differences in component beam light path length through beam splitter 56, photosensor unit 11 is skewed at an angle theta relative to a component beam normal plane such that the total optical path lengths of each of the different color components, as measured from lens 6 to the photosensor unit 11, are equal. Angle theta and the distance "D" between linear photosensor arrays 13, 14 are functions of glass plate and dichroic layer thickness X and index of refraction.
FIG. 3 shows a beam splitter/photosensor arrangement which enables photosensor 11 to be positioned perpendicular to the optical axes of the color-separated beams. In this arrangement, the path-lengths-through-glass of the color-separated beams are made equal by the reciprocal arrangement of trichromatic beam splitters 56 and 58.
As shown in FIG. 3, the incident light beam 4 is aligned to impinge the hypotenuse face 32 of right angle prism 51 at a normal angle and transmit therein to a first base side 30 of the prism 51 which the light beam impinges at 45.degree.. The composite beam splitter 56 of FIG. 1 is attached thereto. A trichromatic separation of the red, green and blue spectral components of the incident light beam occurs as previously described. The three reflected component beams re-enter the prism 51 and are directed toward the second base side 34 of prism 51, each separated beam impinging the second base side 34 at 45.degree. incidence. A second composite beam splitter 58 is attached to the second base side 34 of prism 51. The plates 60 and 62 and the dichroic coatings 50, 52 and 54 in beam splitters 56 and 58 are identical. However, the orientation of the composite beam splitters 56 and 58, and the multilayer dielectric coatings 50, 52 and 54 on each base side 30 and 34 of the prism 51 are reversed so that the path lengths of each component color beam entering and exiting the trichromatic prism beam splitter 59 are identical. That is, a component color beam, such as blue, reflects off the dichroic coating 50 on plate 60 located on base side 30. Next, the blue component reflects off the dichroic coating 50 on plate 60 located adjacent to base side 34. In a like manner, a red component color beam goes from middle filter 52 on base side 30 to middle filter 52 on base side 34, and the green component reflects off a backside filter 54 to a front side filter 54. Reflected beams from the trichromatic beam splitter 58 adjacent to base side 34 are directed out of prism 51. The beams are perpendicular to the hypotenuse side 32 and parallel to the incident light beam. The thickness of the beam splitter glass plates, 60 and 62, and the dichroic coatings, 50, 52 and 54, determine the separation of the reflected beams. Thus, the dual trichromatic beam splitter 59 provides an equal path length through the glass for all color components. Also, the light enters and leaves the prism at a normal angle of incidence.
Referring to FIG. 4, a fluorescent light source 22 illuminates the surface of an original document 21. A beam of imaging light from the original document is projected onto a beam splitter assembly, consisting of dichroic beam splitters 16 and 17, by lens 6. Beam splitters 16 and 17 are flat glass plates coated on one side with dichroic coatings 50 and 52, respectively. Beam splitter 16 is designed to reflect blue light while transmitting red and green spectral bands. The blue light is reflected to a first CCD linear-array photosensor 18, with beam splitter 16 tilted at 45.degree. to the incident light beam 4. Beam splitter 17 reflects red light to a second CCD photodiode array sensor 20. The green line image passing through both beam splitter plates is captured by the third CCD photodiode array sensor 19. Beam splitter plate 17 is also aligned at 45.degree. to the incident light beam 4, as shown. In this arrangement in which each linear photosensor array 18, 19, 20 is provided on a separate photosensor unit differences in optical path lengths of the color component beams through beam splitters 16, 17 are compensated by individually adjusting the positions of the different photosensor units.
U.S. Pat. No. 4,870,268 also discloses a dichroic layer device which comprises a transparent plate having two parallel planar surfaces which is mounted with one of the parallel surfaces positioned in parallel, adjacent relationship with the planar surface of an optical support medium by means of spacers which provide an air gap between the planar surface of the optical support medium and the adjacent planar surface of the plate. The two planar surfaces of the plate and the planar surface of the support medium are each coated with a different dichroic material adapted to reflect different spectral ranges of light. Such a spaced layer arrangement may thus be used to eliminate the need for one of the plates 60, 62 in each of the beam splitter components 56, 58 in a compound beam splitter assembly such as illustrated in FIG. 3.
In the construction of parallel reflective layer-type beam splitters such as illustrated in FIGS. 1 and 3, it is generally desirable, in order to maintain high optical quality in the separated component beams and also to provide a compact beam splitter assembly, to have a relatively small separation between the parallel dichroic layers in each dichroic composite. When a beam splitter is used in a color optical imaging device, it is necessary to provide at least three separate component beams, usually red, green and blue, for proper color imaging. Thus, in prior art beam splitters of the type adapted to produce parallel component light beams, e.g. FIGS. 1 and 3, at least three, parallel, spaced-apart, dichroic layers have been provided. In a compound beam splitter such as illustrated in FIG. 3, each of the component parallel layer beam splitter arrays comprises three spaced-apart dichroic layers. In such arrangements, even if one of the plates in each parallel layer array is replaced by an air gap, it is necessary to employ at least one relatively thin transparent plate for providing mounting surfaces for the different dichroic layers.
However, the use of such thin transparent plates has proved to be problematic. Due to the flexibility of such thin plates, it is difficult to maintain flatness of each surface and parallelism between the three reflective surfaces in a beam splitter composite. When a thin plate is adhered to another surface, discontinuities in the adhesion material tend to produce warping in the attached thin plate. When a thin plate is supported on spacers to provide an air gap between dichroic layers, the lack of rigidity of the plate and the fact that all points on the plate are not supported by the spacers tend to cause warping of the thin plate.
The optical imaging device illustrated in FIG. 3 is adapted to provide focused component images on a plane positioned perpendicular to the component light beams. Such an imaging device requires a total of six separate reflective layers, thus compounding the problems of keeping each of the light reflective layers flat and in proper relationship with the other layers.
Another problem in the construction of composite type beam splitter results from the fact that different spectral ranges of light usually have different indexes of refraction in any given optical medium. As a result, component beams focused by an imaging lens will generally have slightly different focal lengths. Although such chromatic aberration may be corrected through use of a compound lens assembly constructed from a plurality of different materials which are selected to mutually cancel out their individual chromatic aberrations, such lens assemblies are considerably more difficult and more expensive to produce than single medium lens assemblies.
As a result of difficulties experienced in maintaining surface flatness and parallelism in light reflective surfaces and as a result of chromatic aberration produced by ordinary imaging lens assemblies, significant problems are encountered in the construction and assembly of optical imaging devices which employ composite layer type beam splitters and a unitary photosensor unit with coplanar, evenly spaced, linear photosensor arrays. One problem is achieving registration between component beams and corresponding linear photosensor arrays. Another problem is producing a properly focused component image on each linear photosensor array.